Conformal coatings were originally made to protect military and aerospace electronic assemblies from environmental factors such as humidity, dust and chemical contaminants. These environmental factors cause unprotected assemblies with delicate circuitry to malfunction because of corrosion and electrical shorting. As the electronic packaging becomes denser with fine conductor spacings, the assemblies are even more sensitive to the environmental factors. In addition, the increasing use of electronics in transportation and general industrial applications caused the increased use of conformal coatings, mainly for the protection of the electronics from their harsh end-use environments.
The most damaging and usually the most prevalent contaminant generally is considered to be moisture or humidity. Excessive moisture or humidity will drastically lower insulation resistance between conductors, accelerate high-voltage breakdown and dendritic growth, and corrode conductors. Other contaminants which may damage printed circuit boards include various chemicals such as residues of the manufacturing process, e.g. fluxes, organic solvents, release agents, metal particles and marking inks, and contaminants which inadvertently may be deposited by human handling such as body greases, fingerprints, cosmetics and food stains. Ambient operating conditions may also contribute a variety of contaminants such as salt spray, dirt and dust, oil, fuel, acid, corrosive vapor and fungus. In all but the more severe cases, the destructive action of these contaminants effectively can be eliminated by provision of a good conformal coating.
In addition to providing protection from contaminants, conformal coatings also provide a certain degree of protection to mechanical shock, vibration and tampering.
Historically, the first commercial conformal coatings were one and two-part systems whose main constituents were acrylic, epoxy, silicone or polyurethane resins. These resins were usually dissolved in solvents so they could be applied to the parts by spray, dip or brush. These first generation coatings had one or more disadvantages such as the need for mixing before use and required the use of large amounts of solvents to lower the viscosity for application. They also usually required long drying/curing times that released large amounts of volatile organic compounds (VOC) before the coated assemblies could be handled.
Some of these disadvantages were addressed with the introduction of ultraviolet (UV) curable conformal coatings. The UV curable coatings were generally solventless, one-part systems that cured tack-free to the touch, generally via a free-radical polymerization after exposure to UV light. This allowed immediate handling of the coated parts and so reduced the processing time and energy costs, as compared to the solvent based, thermally cured coatings. The application viscosity of these first generation UV curable coatings tended to be higher than solvent based systems. Also being solventless, the viscosity could not be easily adjusted which makes applying thin films more difficult.
The rapid transformation of a liquid composition into a crosslinked solid happened only to the areas exposed to the UV light and not in the shadowed area (e.g., under components). This shadowing became more of a problem as electronic assemblies with high component densities became more common. Therefore, a secondary cure mechanism such as heat was sometimes incorporated into acrylate, UV curable conformal coatings to polymerize these shadowed areas while maintaining one-part stability. But thermal cure schedules of.gtoreq.100.degree. C. were required to complete this secondary curing process.
Various dual-curing conformal coating systems are known in the art, with each having its advantages and disadvantages.
For instance, two component dual curing systems, such as two component polyurethane systems, offer short curing times. See U.S. Pat. No. 4,424,252 to Nativi et al. In particular, the '252 patent to Nativi discloses a two component system comprising reacting a urethane acrylate with an aliphatic or aromatic polyisocyanate adduct. The coating the cures within 2 hours to a day through several mechanisms, one of which includes reaction between free isocyanates on the polyisocyanate adduct and atmospheric moisture. The curing mechanisms also include reaction between the isocyanate groups and the hydroxyl groups on the urethane acrylate. As a result of the latter reaction, however, this system, as well as other two component systems, e.g. see U.S. Pat. No. 5,013,631 to Su, have a limited pot life (to about 48 hours or less), and thus forces the user to carry out a mixing step just prior to use.
One component systems avoid the pot life problems incurred by the two-component systems in that they avoid including any isocyanate reactive functional groups in the system. See U.S. Pat. No. 4,415,604 to Nativi. For instance, the '604 patent to Nativi discloses a one-component system which comprises an isocyanate-capped polyether diol and triol, an acrylate diluent and a photoinitiator. However, because the isocyanate-capped diols and triols are based on hydrophilic polyethers, the resulting coating loses some hydrolytic stability and resistance to moisture. Thus, in more humid environments, the coating is not as effective.
Another type of dual-curing one-component conformal coating involves a curing mechanism other than an isocyanate reaction. For instance, U.S. Pat. No. 4,451,523 to Nativi et al. discloses a one-component composition comprising a urethane-(meth)acrylate, an allyl-group containing (meth)acrylate monomer, a nonallylic (meth)acrylate diluent, photoinitiator and a metal drier. The crosslinking of the allylic compounds in the presence of metal driers provides the second curing mechanism in addition to UV curing of a (meth)acrylate. However, this system may contribute metal ionic species which reduce the electrical properties of the coating. These species could also cause degradation of electronic components by promoting an electrical pathway between closely packed components.
It is also known to employ dual curing coatings in areas other than electronics. For instance, radiation dual curable coatings have been employed in the automobile industry. See U.S. Pat. No. 4,173,682 to Noomen et al. In particular Noomen et al. disclose a two-component automobile coating system comprising (i) an isocyanate group-containing adduct prepared from a (meth)acrylic hydroxy ester and a polyisocyanate and (ii) a polyfunctional hydroxy compound. However, not only does this two-component system suffer from the disadvantages discussed above in terms of the electronic coatings, it also apparently suffers from long ambient cure times, e.g., a few days, when compared to cure times for other two-component systems. In addition, some of the isocyanates disclosed by Noomen et al., e.g., the adduct of hexamethylene diisocyanate and water, contain biuret linkages. Compounds containing these linkages are not the most thermal or "weather" stable. By "weather stable", it is meant stability under ambient humidity, temperature fluctuations and sunlight.
U.S. Pat. No. 4,138,299 to Bolgiano discloses a one-component dual Curing composition suitable for glossy, abrasion resistant floor coatings. Specifically Bolgiano discloses a one-component composition consisting essentially of an isocyanate (NCO) terminated prepolymer (prepared from an aliphatic isocyanate and a polyester diol and triol), and an acrylate diluent which contains no reactive hydroxyl groups. The NCO terminated prepolymer is also further reacted with sufficient hydroxy acrylate to cap 5 to 15% of the available NCO groups. The composition primarily cures through crosslinking of the acrylates' ethylenically unsaturated groups and secondarily through chain extension and crosslinking of the free NCO groups and water. However, as with the composition disclosed in the '604 patent to Nativi, Bolgiano's composition contains hydrophilic moieties, i.e. the polyester linkages. Accordingly, in humid environments the hydrolytic stability of the composition is not as effective.
As evident from the above discussion and as is well known in the art, no dual curing conformal coating is completely satisfactory for all applications due to the varying processing (e.g. processing speed, pot life and cure conditions) and end use demands (e.g. thermal characteristics, weathering and hydrolytic stability). Accordingly the intended application and the environment related to that application have typically dictated the particular formulation of the coating.
The present invention is a new dual UV/moisture curable conformal coating composition which is a solventless, one-part storage stable, low viscosity liquid that rapidly crosslinks into a tack-free solid when exposure to UV light. This UV cure allows the coated part to be immediately handled and prevents run off of the liquid coating trapped in the shadowed areas. The coating in the shadowed areas is polymerized to a dry-to-the-touch solid upon standing in the open by a reaction with the atmospheric moisture.
An added advantage of the present invention is that it is amendable to varying process demands. For example, it is forgiving of high humidity conditions. That is, its static pot life is somewhat tolerant of ambient moisture and heat in the production environment. The UV cure need not be completed to produce the tack-free surface. Rather, the exposure need only be sufficient to initiate cure. The initial surface tack will be eliminated upon standing. Thus it is possible to decrease the duration and/or intensity of UV exposure of the articles, if desired. The longer cure time and low viscosity of the liquid can contribute to a reduction in product defects. Carbon dioxide can form during the curing step as a byproduct of the polymerization reaction. If the coating cures a bit more slowly and the initial viscosity is low, this gas has more time to diffuse out of the coating instead of forming bubbles. Further, there is more time for inspection and repair of defects in the assemblies during production. Finally, a thinner, more even coating might be obtained.